The FM band (i.e., a frequency band between about 76 and about 108 MHz) is used worldwide. FM-band transmitters find broad application in many types of wireless devices, including mobile cellular devices.
The Q factor, or quality factor, is a dimensionless ratio relating the resonant frequency of a device or circuit to its power dissipation; the higher the Q factor, the greater the efficiency. The Q factor of a loop antenna, such as may be printed on a printed circuit board, can be high. However, its Q factor decreases as frequency decreases. A quarter-wavelength FM-band printed loop antenna would have to be about 75 cm long, which is impractical for mobile cellular devices. The efficiency of a loop antenna that is practical in such devices is relatively low for the FM band.
While the Q factor is low, it still varies materially over the 76 MHz to 108 MHz range of the FM band. This causes the power of the transmitted signal to vary from one end of the band to the other, which is disadvantageous. Unfortunately, decreasing the antenna's Q factor to stabilize the power decreases its efficiency and increases its power dissipation. This is disadvantageous in battery-powered devices.